Project Summary To counter the accumulation of DNA damage, eukaryotic cells employ an intricate network of pathways that promote damage recognition, checkpoint signaling, and DNA repair. Components of the damage response network have been linked to various genetic disorders that are typified by hypersensitivity to DNA damaging agents and cancer predisposition. In particular, the tumor suppressor BRCA1 has been described as a master regulator of genome stability due to its involvement in various aspects of the damage response. Despite BRCA1's established role in the development of breast and ovarian cancers, a molecular understanding of its functions remains limited. We recently showed that Xenopus egg extracts recapitulate BRCA1 functions observed in human and mouse models, providing a soluble, highly amenable system for future mechanistic studies. Initially, we will focus on BRCA1's newly described role in dismantling the replicative helicase after it collides with a DNA crosslink. Unloading the CMG helicase complex (comprised of Cdc45, MCM2-7, and GINS) is required to access and repair the underlying lesion, suggesting that helicase eviction may be a fundamental mechanism of DNA damage tolerance. We will also identify how BRCA1's functions are connected to cellular sensitivity, enabling us to devise and test new strategies for improved crosslink-based therapeutics. Going forward, we will extend our analysis to additional BRCA1 functions to establish a comprehensive understanding of BRCA1's many roles in DNA repair and tumor suppression.